This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-086158, filed Mar. 23, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a distance information acquisition apparatus or system, a distance information acquisition method, and a pattern projector used for these configured to capture an image of an object onto which a specified pattern is projected, to analyze the captured image, and to acquire information on the distance to the object.
2. Description of the Related Art
As a conventional method of measuring the distance to an object to be measured, there is known a pattern projection method of projecting a non-repetitive pattern onto the object to be measured. This method acquires a 3-D (three-dimensional) shape by means of pattern projection.
According to this method, a projector projects a specified pattern onto an object to be measured. An image capturing element (camera) captures that image. A computer analyzes this image to obtain 3-D data for the object, i.e., distance information to each part of the object. More specifically, pattern P as shown in FIG. 1 is printed on a film. The pattern P is projected by placing this film between the projector""s lighting optical system including a light source and the object. In this example, pattern P comprises nine pattern segments. The hatching in FIG. 1 shows that the respective pattern segments differ from each other in optical characteristics such as colors, gradations, designs, etc.
The need for more accurately measuring distances to more parts increases the number of pattern segments (the number of stripes in this example) in a projection pattern. In addition, many halftones (colors) need to be used.
As mentioned above, it is necessary to increase the number of projection pattern elements (pattern segments or stripe designs) in order to increase points for measuring distances and obtain accurate 3-D information (distance information). This decreases the difference in colors or gradations between adjacent pattern segments. Accordingly, the following problems are expected when an image is captured or quantized from an object onto which such a projection pattern is projected.
(1) An attempt to increase the accuracy or resolution necessitates projecting a pattern having fine, many-valued gradations (colors), decreasing a difference between pattern segments. Consequently, a quantization error easily occurs, finally causing a ranging error.
(2) The production of a projection pattern requires improving the accuracy for reproducing color tones or gradations. Accordingly, it is highly possible that manufacturing processes are complicated and the management or calibration becomes difficult.
The present invention has been made in consideration of the foregoing. It is therefore an object of the present invention to provide a distance information acquisition apparatus or system, a pattern projector, and a distance information acquisition method capable of preventing a ranging error, simplifying manufacturing processes, and facilitating the management or calibration.
According to a first aspect of the present invention, there is provided a distance information acquisition apparatus which captures an object with a specified pattern projected, analyzes the captured image, and obtains distance information associated with the object, the apparatus comprising:
a pattern projection module which projects a pattern onto the object, wherein the pattern is formed by combining the element groups (i.e., sub-patterns), each element group (i.e., sub-pattern) comprising at least three types of elements (i.e., pattern segments) having optically identifiable characteristics;
an image capturing module which is placed a specified distance from the pattern projection module and captures the image of an object having the pattern projected; and
a correspondence specifying module configured to determine which element (i.e., pattern segment) in a captured image taken by the image capturing modules belongs to which element group (i.e., sub-pattern) in the pattern based on optical characteristics and position coordinates of the element (i.e., pattern segment) in the captured image, wherein
a specification result of the correspondence specify module is used to find distance information associated with the object.
According to a second aspect of the present invention, there is provided a pattern projector used for a distance information acquisition apparatus which captures the image of an object with a specified pattern projected, analyzes the captured image, and obtains distance information associated with the object, the projector comprising:
a film having a pattern printed thereon, wherein the pattern is formed by combining the element groups (i.e., sub-patterns), each element group (i.e., sub-pattern) comprising at least three types of elements (i.e., pattern segments) having optically identifiable characteristics; and
a pattern projection module configured to project the pattern onto the object.
According to a third aspect of the present invention, there is provided a distance information acquisition method of capturing the image of an object with a specified pattern projected, analyzing the captured image, and obtaining distance information associated with the object, the method comprising:
projecting a pattern onto the object, wherein the pattern is formed by combining the element groups (i.e. sub-patterns), each element group (i.e., sub-pattern) comprising at least three types of elements (i.e., pattern segments) having optically identifiable characteristics;
capturing the image of the object having the pattern projected thereon by an image capturing module placed with a specified distance from a pattern projection module configured to project the pattern;
determining which element (e.g., pattern segments) in a captured image taken by the image captured module belongs to which element group (e.g., sub-pattern) in the pattern based on optical characteristics and position coordinates of the element in the captured image; and
using the specify result to find distance information associated with the object.
According to a fourth aspect of the present invention, there is provided a distance information acquisition apparatus or system which captures the image of an object with a specified pattern projected, analyzes the captured image, and obtains distance information associated with the object, the apparatus or system comprising:
a pattern projection module which projects a pattern onto an object, wherein the pattern comprises at least three types of elements (i.e., pattern segments) having optically identifiable characteristics and includes a plurality of the elements (i.e., pattern segments) of at least one type;
an image capturing module which is placed with a specified distance from the pattern projection module and captures the image of an object having the pattern projected; and
a correspondence specifying module configured to determine which element (i.e., pattern segment) in a captured image taken by the image capturing module corresponds to which element (i.e., pattern segment) in the pattern based on optical characteristics and position coordinates of the element (i.e., pattern segment) in the captured image, wherein
when the pattern is projected onto a plane including a farthest ranging limit, a distance from an occurrence of one of the plurality of elements (i.e., pattern segments) to a point just before the next occurrence of an element (i.e., pattern segment) of the same type is greater than or equal to a minimum value determined on the basis of a distance measuring range.
In the above-mentioned first to fourth aspects, the above-mentioned pattern is formed by arranging the above-mentioned elements (i.e., pattern segments) along the direction of a baseline which links the above-mentioned pattern projection module and the above-mentioned image capturing module, and by repeating the above-mentioned element group (i.e., sub-pattern).
The above-mentioned pattern may be configured to provide part of the above-mentioned element group (i.e., sub-pattern) with marks for specifying respective element groups (i.e., sub-patterns).
Alternatively, the above-mentioned pattern may be configured by arranging the above-mentioned elements (i.e., pattern segments) along the direction of a baseline which links the above-mentioned pattern projection module and the above-mentioned image capturing module, and by comprising element groups (i.e., sub-patterns) with a plurality of different characteristics.
The above-mentioned pattern may be projected onto a plane equivalent to the farthest measuring limit of the above-mentioned distance information acquisition apparatus. In this case, the width of the above-mentioned element group (i.e., sub-pattern) should be larger than or equal to a minimum width determined by a measurable range (i.e., from the nearest ranging limit to the farthest ranging limit).
Moreover, the above-mentioned pattern may be projected onto a plane equivalent to the farthest ranging limit of the above-mentioned distance information acquisition apparatus. In this case, it may be preferable to further provide a change module configured to change the width of the above-mentioned element group (i.e., sub-pattern) according to a measurable range (i.e., from the nearest ranging limit to the farthest ranging limit).
The above-mentioned element group (i.e., sub-pattern) preferably comprises high-brightness and high-saturation colors.
As mentioned above, the present invention can increase a difference between pattern segments by repetitively using sub-patterns. It is possible to suppress occurrence of a quantization error due to the use of many halftones as conventionally practiced and to prevent a ranging error due to erratic detection of codes.
It is possible to minimize the sub-pattern width and the number of sub-pattern repetitions. This can accurately measure distances with the minimum number of codes (e.g., the number of gradations or colors) by suppressing erratic detection of codes.
Since the number of gradations or colors decreases, pattern generation means (i.e., film having a pattern printed thereon) can be manufactured easily.
Furthermore, since the number of gradations or colors decreases, it is possible to decrease the amount of computation for a computer as a control and analysis apparatus. Fast processing can be expected.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.